High carbon tool steels by powder metallurgy

ABSTRACT

THIS APPLICATION RELATES TO THE POWDER METALLURGY OF WROUGHT, HIGH CARBON TOOL STEELS AND ALSO TO A POWDER METALLURGY METHOD FOR PRODUCING SAID STEELS CHARACTERIZED METALLOGRAPHICALLY BY A UNIFORM DISTRIBUTION OF FINELY DIVIDED CARBIDES IN BOTH THE LONGITUDINAL AND TRANSVERSE DIRECTIONS.

July 6, 1971 J. s. BENJAMIN 3,591,349

HIGH CARBON TOOL STEELS BY POWDER METALLURGY Filed Aug. 27, 1969 UntedStates Patent O 3,591,349 HIGH CARBON TOOL STEELS BY POWDER METALLURGYJohn Stanwood Benjamin, Sutfern, N.Y., assigner to The InternationalNickel Company, Inc., New York, N.Y. Continuation-impart of applicationSer. No. 709,700, Mar. 1, 1968. This application Aug. 27, 1969, Ser.

Int. Cl. C22c 39/ 54 U.S. Cl. 29-182.7 7 Claims ABSTRACT OF THEDISCLOSURE This application relates to the powder metallurgy of wrought,high carbon tool steels and also to a powder metallurgy method forproducing said steels characterized metallographically by a uniformdistribution of finely divided carbides in both the longitudinal andtransverse directions.

This application is a continuation-in-part of U.S. application Ser. No.709,700, filed Mar. l, 1968.

THE RELATED APPLICATION In the aforementioned related application, Ser.No. 709,700, which is incorporated herein by reference, a method isdisclosed for producing a wrought composite metal powder comprised of aplurality of constituents mechanically alloyed together, at least one ofwhich is a metal capable of being compressively deformed such thatsubstantially each of the particles is characterized metallographicallyby an internal structure comprised oi the starting constituentsintimately united together and identifiably mutually interdispersed. Oneembodiment of a method for producing the composite powder resides inproviding a dry charge of attritive elements and a powder masscomprising a plurality of constituents, at least one f which is a metalwhich is capable of being compressively deformed. The charge issubjected to agitation milling under high energy conditions in which asubstantial portion or cross section of the charge is maintainedkinetically in a highly activated state of relative motion and themilling continued to produce wrought composite metal powder particles ofsubstantially the same composition as the starting mixture characterizedmetallographically by an internal structure in which the constituentsare identifiable and substantially mutually interdispersed withinsubstantially each of the particles. The internal uniformity of theparticles is dependent on the milling time employed. By using suitablemilling times, the interparticle spacing of the constituents within theparticles can be made very small so that when the particles are heatedto an elevated diffusion temperature, interdilfusion of difusibleconstituents making up the matrix of the particle is effected quiterapidly.

Tests have indicated that the foregoing method enables the production ofmetal systems in which insobluble nonmetallics such as refractoryoxides, carbides, nitrides, silicides, and the like, can be uniformlydispersed throughout the metal particle. In addition, it is possible tointerdisperse alloying ingredients within the particles, particularlylarge amounts of alloyng ingredients, eg., such as chromium, which havea propensity to oxidize easily due to their rather high free energy offormation of the metal oxide. In this connection, mechanically alloyedpowder particles can be produced by the foregoing method containing anyof the metals normally difficult to alloy with another metal.

THE PRIOR ART High carbon tool steels are produced conventionally bymelting, casting of the molten metal into an ingot, and

ice

then, after subjecting the ingot to the usual soaking treatment at anelevated temperature followed by surface cleaning, hot working the ingotby stages to the desired shape. Freezing of an ingot plays an importantpart in the quality of the final product, namely, segregation. Complexalloys, particularly high carbon, high alloy tool steels may suffer fromseveral kinds of segregation which can have an adverse effect on theforgeability of the ingot and on its metallographic structure. In themolten condition, high carbon alloy steels are partically uniformthroughout, the Various elements present, such as carbon, silicon,manganese, chromium, vanadium, tungsten, molybdenum, and the like, beingessentially dissolved. Since ingots produced in practice are generallylarge, selective solidication takes place during freezing, leading tocomposition segregation along the length and the width of the ingot.Unless this is removed by subsequent treatment, the alloy may exhibitnon-uniform response to heat treatment.

Moreover, during freezing, large dendrites and carbide segregates and/or aggregates may form in the ingot. After solidication of the ingot,such segregates and/or aggregates may be broken up with difficulty bymechanical work. However, such carbides are generally brittle andadversely affect the ductility of the ingot. Where the formation ofcarbide segregates and aggregates is rather marked in the ingot, thecarbide distribution appears as elongated structures in the forged orhot Worked product in the longitudinal direction with areas therebetweenimpoverished in carbides. Such metallurgical structures are notdesirable and can have an adverse effect on the physical properties.

Attempts have been made to utilize powder metallurgy techniques as analternative method for producing high carbon tool steels free fromcoarse dendrites and segregates characteristic of the conventionalmelting and casting practice. However, it has been difiicult to obtaingood composition homogeneity by solid state difliusion at elevatedtemperatures due to diffusion sluggishness of such alloying ingredientsas chromium, tungsten, molybdenum, etc., in the powder condition. Whilethe diffusion path of the elements can be decreased by using very smallparticle sizes, for example 2 or 1 micron powder, such sizes tend to bepyrophoric and hence easily subject to contamination by reaction withthe environment, e.g., air.

While many attempts have been made to produce high carbon high alloytool steel having optimum composition uniformity, having good responseto heat treatment, and having a uniform dispersion of finely dividedcarbide throughout the steel, none, as far as I am aware, has beenwholly successful prior to the present invention.

It is thus an object of this invention to provide a powder metallurgymethod for producing a wrought, high carbon tool steel characterized bya high degree of composition uniformity and by optimum response to heattreatment.

Another object is to provide a powder metallurgy method for producing aWrought, high carbon high alloy tool steel product in whichcontamination during the early stages of manufacture is not a problem.

A further object is to provide a powder metallurgy method for producinga high carbon, high alloy tool steel characterized metallographically bya uniform dispersion of finely divided carbide and being free of carbidesegregates and/or aggregates.

This invention also provides as an object a powder metallurgy producedwrought high carbon tool steel characterized by a high degree ofcomposition uniformity, by optimum response to heat treatment, by auniform dispersion of nely divided carbide and further characterized inbeing substantially free from carbide segregates and/or aggregates.

These and other objects will more clearly appear when taken inconjunction with the following description and the accompanying drawing,wherein:

FIG. l depicts schematically a portion of a ball charge in a kineticstate of random collision; and

FIG. 2 is a schematic representation of an attritor of the stirred ballmill type capable of providing agitation milling to produce compositemetal particles employed in carrying out the invention.

STATEMENT OF THE INVENTION In its broad aspects, the present inventionis directed to the powder metallurgy production of a wrought high carbontool steel product characterized substantially by uniform compositionthroughout, by a uniform dispersion of finely divided carbide, andfurther characterized in being substantially free from carbidesegregates and/or aggregates. In its more preferred aspects, theinvention provides a high carbon tool steel product characterized by ahigh degree of carbide dispersion substantially free from carbidesegregates and/or aggregates in both the longitudinal and transversecross sections and, particularly, in any selected area when viewed inmagnification of up to 10,000 times or more. Such uniformity resultsfrom the use of a dense, wrought, metal composite particle having ahighly uniform internal structure. In other words, by starting with theforegoing composite particles as the building blocks in producing thewrought metal shape, the high degree of uniformity of each of thecomposite particles is carried forward and maintained in the nal wroughtproduct with substantially no carbide segregates and/or aggregates inthe internal structure.

For the purposes of this invention, it is advantageous that the productcontains less than l volume percent of segregated regions exceeding 25microns in minimum dimension. A segregated region is one in which thereis a significant composition fluctuation exceeding 25 microns or even l0microns, in size. A significant composition fluctuation is defined as adeviation exceeding of the mean content of the alloying element present.The size of the segregated regions is measured as the minimum distancethrough the segregated region between adjacent positions bounding thesegregated region at a composition deviation one-half of the maximumuctuation of the amount present.

The wrought composite metal particles which are employed in the startingmaterial are defined in copending application Ser. No. 709,700 as beingmade by integrating together into dense particles a plurality ofconstituents in the form of powders, at least one of which is acompressively deformable metal. The requirement of deformable metal isfulfilled by iron since it constitutes essentially the balance of thesteel composition. In one method, the constituents are intimately unitedtogether to form a mechanical alloy within individual particles withoutmelting any one or more of the constituents. Thus, the formation ofcarbide segregates, dendrites and/or aggregates is substantiallyavoided. By the term mechanical alloy is meant that state which prevailsin a composite metal particle wherein a plurality of constituents in theform of powders, at least one of which is a compressively deformablemetal, are caused to be bonded or united together, according to onemethod, by the application of mechanical energy in the form of aplurality of repeatedly applied compressive forces sufficient tovigorously work and deform at least one deformable metal and cause it tobond or weld t0 itself and/or to the remaining constituents, be theymetals and/or non-metals, whereby the constituents are intimately unitedtogether. By repeated fracture and rewelding together' of the compositeparticles thus formed a fine codissemination of the fragments of thevarious constituents throughout the internal structure of each particleis achieved. Concurrently, the overall particle size distribution of thecomposite particles remains substantially constant throughout theprocessing. By observation of the grinding media, c g., balls, duringprocessing, it appears that the major site at which Welding andstructural refinement of the product powder takes place is upon thesurfaces of the balls.

The process employed for producing mechanically alloyed particlescomprises providing a mixture of a plurality of powdered constituents,at least one of which 1s a compressively deformable metal, and at leastone other constituent is selected from the group consisting of anonrnetal and another chemically distinct metal, and subjecting themixture to the repeated application of compressive forces, for example,by agitation milling as one method under dry conditions in the presenceof attritive elements maintained kinetically in a highly activated stateof relative motion, and continuing the dry milling for a time suicientto cause the constituents to comminute and bond or weld together andcodisseminate throughout the resulting metal matrix of the productpowder. The mechanical alloy produced in this manner is characterizedmetallographically by a cohesive internal structure in which theconstituents are intimately united to provide an interdispersion ofcomminuted fragments of the starting constituents. Generally, theparticles are produced in a heavily cold worked condition and exhibit amicrostructure characterized by closely spaced striations.

It has been found particularly advantageous in obtaining optimum resultsto employ agitation milling under high energy conditions in which asubstantial portion of the mass of the attritive elements is maintainedkinetically in a highly activated state of relative motion. However, themilling need not be limited to such conditions so long as the milling issufficiently energetic to reduce the thickness of the initial metalconstituents to less than one-half of the original thickness and, moreadvantageously, to less than of the average initial particle diameterthereof by impact compression resulting from collisions with the millingmedium, eg., grinding balls.

As will be appreciated, in processing powder in accordance with theinvention, countless numbers of individual particles are involved.Similarly, usual practice requires a bed of grinding media containing alarge number of individual grinding members, e.g., balls. Since theparticles to be contacted must be available at the collision sitebetween grinding balls or between grinding balls and the wall of themill or container, the process is statistical and time dependent.

By the term agitation milling, or high energy milling is meant thatcondition which is developed in the mill when sufficient mechanicalenergy is applied to the total charge such that a substantial portion ofthe attritive elements, e.g., ball elements, are continuously andkinetically maintained in a state of relative motion with each other;that is to say, maintained kinetically activated in random motion sothat a substantial number of elements repeatedly collide with oneanother. It has been found advantageous that at least about e.g., 50% oror even 90% or more, of the attritive elements should be maintained in ahighly activated state,

Since generally the composite metal particles produced in accordancewith the invention exhibit an increase in hardness with milling time, ithas been found that, for purposes of this invention, the requirements ofhigh energy milling are met when a powder system of carbonyl nickelpowder mixed with 2.5 volume percent of thoria is milled to providewithin hours of milling and, more advantageously, within 24 hours, acomposite metal powder whose hardness increase with time is at leastabout 50% of substantially the maximum hardness increase capable ofbeing achieved by the milling. Putting it another way, high energymilling is that condition which will achieve in the foregoing powdersystem an increase in hardness of at least about l/2 of the differencebetween the ultimate saturated hardness of the composite metal particleand its base hardness, the base hardness being that hardness determinedby extrapolating to zero milling time a plot of hardness data obtainedas a function of time up to the time necessary to achieve substantiallymaximum or saturation hardness. The resulting composite metal particlesshould have an average particle size greater than 3 microns and, moreadvantageously, greater than microns, with preferably no more than 10%by weight of the product powder less than one micron.

By maintaining the attritive elements in a highly activated state ofmutual collision in a substantially dry environment and throughoutsubstantially the whole mass, optimum conditions are provided forcomminuting and cold welding the constituents accompanied by particlegrowth, particularly with reference to the finer particles in the mix,to produce a mechanically alloyed structure of the constituents withinsubstantially each particle. Where at least one of the compressivelydeformable metallic constituents has an absolute melting pointsubstantially above about 1000 K., the resulting composite metal powderwill be heavily cold worked due to impact compression of the particlesarising from the repeated collision of elements upon the metalpatricles. For optimum results, an amount of cold work foundparticularly useful is that beyond which further milling does notfurther increase the hardness, this hardness level having been referredto hereinbefore as saturation hardness. This saturation hardness istypically far in excess of that hardness obtainable in bulk metals ofthe same composition by such conventional working techniques as coldforging, cold rolling, etc. The saturation hardness achieved in purenickel processed in accordance with this invention is about 477 kg./mm.2as measured by a Vickers microhardness tester, while the maximumhardness obtained by conventional cold working of bulk nickel is about250 kg./mm.2. The values of saturation hardness obtained in processingalloy powders in accordance with this invention frequently reach valuesbetween 750 and 850 kg./mm.2 as measured by Vickers microhardnesstechniques. Those skilled in the art will recognize the amazingmagnitude of these figures. The saturation hardness obtained in powdersprocessed in accordance with this invention is also far in excess of thehardnesses obtained in any other process for mixing metal powders.

As illustrative of one type of attritive condition, reference is made toFIG. l which shows a batch of ball elements 10 in a highly activatedstate of random momentum by virtue of mechanical energy appliedmultidirectionally as shown by arrows 11 and 12, the transitory state ofthe balls being shown in dotted circles. Such a condition can besimulated in a vibratory mill. Another mill is a high-speed shaker milloscillated at rates of up to 1200 cycles or more per minute whereinattritive elements are accelerated to velocities of up to about 300centimeters per second (cm./sec.).

A mill found particularly advantageous for carrying out the invention isa stirred ball mill attritor comprising an axially vertical stationarycylinder having a rotatable agitator shaft located coaxially of the millwith spaced agitator arms extending substantially horizontally from theshaft. A mill of this type is described in the Szegvari U.S. Pat. No.2,764,359 and in Perrys Chemical Engineers Handbook, fourth edition,1963, at pages 8-26. A schematic representation of this mill isillustrated in FIG. 2 of the drawing which shows in partial section anupstanding cylinder 13 surrounded by a cooling jacket 14 having inletand outlet ports 15 and 16, respectively, for circulating a coolant,such as Water. A shaft 1'7 is coaxially supported within the cylinder bymeans not shown and has horizontal extending arms 18, 19 and 20 integraltherewith. The mill is filled with attritive elements, eg., balls 21,suliicient to bury at least some of the arms so that, when the shaft isrotated, the ball charge, by virtue of the agitating arms passingthrough it, is maintained in a continual state of unrest or relativemotion throughout the bulk thereof.

The dry milling process of the invention is statistical and timedependent as well as energy input dependent, and milling isadvantageously conducted for a time sufficient to secure a substantiallysteady state between the particle growth and particle comminutionfactors. If the specific energy input rate in the milling device is notsuflicient, such as prevails in conventional ball milling practice forperiods up to 24 or 36 hours, a compressively deformable powder willgenerally not change in apparent particle size. It is accordingly to beappreciated that the energy input level should advantageously exceedthat required to achieve particle growth, for example, by a factor of 5,10, or 25, such as described for the attritor mill hereinbefore. In suchcircumstances, the ratio of the grinding medium diameter to the averageparticle diameter is large, e.g., at least 20 times or more. Thus, usingas a reference a mixture of carbonyl nickel powder having a Fishersubsieve size of about 2 to 7 microns mixed with about 2.5% by volume ofless than 0.1 micron thoria powder, the energy level in dry milling inthe attritor mill, e.g., in air, should be sufficient to provide amaximum particle size in less than 24 hours. A mill of the attritor typewith rotating agitator arms and having a capacity of holding one gallonvolume of carbonyl nickel balls of plus 1/21 inch and minus 1/2 inchdiameter with a ball-topowder volume ratio of about 20 to l, and withthe impeller driven at a speed of about 180 revolutions per minute(r.p.m.) in air, will provide the required energy level.

The milling time t required to produce a satisfactory dispersion; theagitator speed W (in r.p.m.); the radius, r, of the cylinder (in cm.)and the volume ratio R of balls to powder are related by the expression:

where K is a constant depending upon the system involved. Thus, once aset of satisfactory conditions has been established in one mill of thistype, other sets of satisfactory conditions for this and other similarmills may be predicted by use of the foregoing expression. When drymilled under these energy conditions without replacement of the airatmosphere, the average particle size of the reference powder mixturewill increase to an average particle size of between about to 125microns in about 24 hours. A conventional ball mill generallyaccomplishes a mixing of the Ipowders with some incidental flattening ofthe nickel powders and negligible change in product particle size afterup to 24 or 36 hours grinding in air.

Attritor mills, vibratory ball mills, planetary ball mills, and someball mills depending upon the ball-to-powder ratio and mill size, arecapable of providing energy input within a time period and at a levelrequired in accordance with the invention. In mills containing grindingmedia, it is preferred to employ metal or cermet elements or balls, eg.,steel, stainless steel, nickel, tungsten carbide, etc., of relativelysmall diameter and of essentially the same size. The volume of thepowders being milled should be substantially less than the dynamicinterstitial volume between the attritive elements, e.g., the balls,when the attritive elements are in an activated state of relativemotion. Thus, referring to FIG. l, the dynamic interstitial volume isdefined as the sum of the average volumetric spaces S between the ballswhile they are in motion, the space between the attritive elements orballs being sufficient t0 allow the attritive elements to reachsufticient momentum before colliding. In carrying out the invention, thevolume ratio of attritive elements to the powder should advantageouslybe over about 4 to l and, more advantageously, at least about l0 to l,so long as the volume of Ipowder does not exceed about one-quarter ofthe dynamic interbe produced by the invention, Tables I and 1I aregiven. Iron is not listed in the tables, it being essentially thebalance of each of the compositions listed.

TABLE I Nominal composition, percent by weight Type steel tliromiuni i(llromiuin-lnolylidenuni 'Iungstcn-linishing steel Semihigh speed steelsAir-hardcning die .steels liigh carbon, high chromium dit* stccls Wearresistant die steels Special wear resistant die steel TABLE II Nominalcomposition, percent by weight 'l`.\ pc steel C Mn Si Cr Ni V W Mo Co 85l 1f-4. 25 2f2. 15 18-18. 5 0. 5-0. 75 Tungsten types U8 0 1-0. 4 0 l-U.l Atf-4. 25 2-2. 15 18-18. 5 0. 5-0. 75 03 i 3. 75-4. 25 2. 8e3. 2 13.5-14. 5 0. (S5-0. 5 "5 l -14.5 1. 0-l. 25 el!) G-O. 8 Tungsten-cobalttypes .t .r 0 1-0. 4 0. 1-0. 4 4 5-4. T5 4. 7545. 0 12. 513. 5 0. 4-0. 6o. s5 I 4. o4. 5 1.6-2. o 1s. 75-20.G 5 o. ts-g. s -1185 l1.25 1. 5-1.V5 -9 Molybdenum types 8 1.03 0. 1-0. 4 0. 1-0. 4 3. 15-4. 0 l', l 1 5 175 8 5 8 7g x A t l). 84). b5 l l 3. "5x-1. 25 1. 1-1. 4 l. 5*-1. 8 8.25-S. M015 bdcnunrcobalt t5 pcs 0. T u U3 s 0.1-0.4 0. 1-0. 4 l 5 0 l8&2. 25 1V B Log s.

ypCS 1.5-1.6 0. 1-114 0. 10.4 4.0-4.75 4. 755. 25 6. 25-6. 75 3.0-5 04.75*5.25 Self-hardening type 2. 25 1 5 0. 25 2. 0 i. 11. 0

The deformable metals in the mixture are thus subjected to a continualkneading action by virtue of impact compression imparted by the grindingelements, during which individual metal components making up thestarting powder mixture become comminuted and fragments thereof areintimately united together and become mutually in terdispersed to formcomposite metal particles having substantially the average compositionof the starting mixture.

The product powders produced in accordance with the invention have theadvantage of being non-pyrophoric, i.e., of not being subject tospontaneous combustion when exposed to air. Indeed, the product powdersare sucient- 1y large to resist substantial surface contamination whenexposed to air.

The product particles may have a size of up to about 500 microns with aparticle size range of about 3 to about 200 microns being more commonwhen the initial mixture contains a major proportion of an easilydeformable metal, such as iron. The relatively large particle size andlow surface area which characterize the composite particles is anoutstanding advantage in powder metallurgy processes requiring vacuumdegassing for removing adsorbed or absorbed gases. The significance ofthis advantage becomes particularly marked when it is considered thatcertain ine metal particles absorb as much as l0 times the volume of gaspresent in the interstitial spaces between the powder particles.

DETAIL ASPECTS OF THE INVENTION The foregoing procedure is particularlyapplicable to the production of high carbon alloy tool steels and, inparticular, high carbon, high alloy tool steels.

As stated hereinbefore, the powder mixture may comprise a plurality ofconstituents so long as at least one is compressively deformable. Inorder to produce the desired composite particles, the ductible metalshould comprise at least about 15%, or 25% or 50% or more by volume ofthe total composition. Where two or more compressive- 1y deformablemetals are present. it is to be understood that these metals togethershould comprise at least about 15% by volume of the total composition.

As examples of Wrought high carbon tool steels that can Broadly stated,the compositions covered by this invention range by weight from about0.7% to 4% carbon, at least about 0.1% of at least one alloying elementfrom the group consisting of chromium, vanadium, tungsten and molybdenumand the balance essentially iron, for example, about 40%, 45% or 50% ormore iron.

A composition range to which the invention is particularly applicable isone containing about 0.9% to 3.5% carbon` at least about 1% of at leastone alloying element selected from the group consisting of chromium,vanadium, tungsten and molybdenum, and the balance essentially iron, forexample, about 40% or 45% or 50% Or more iron.

Another composition range is one containing about 0.9% to 3% or 3.5%carbon, about 3% to 15% chromium, up to about 10% or 20% vanadium, up to25% tungsten, up to about 12% molybdenum, and the balance essentiallyiron.

A composition particularly advantageous in producing high carbon toolsteels of the chromium-vanadium-tungsten variety, including the highspeed steel varieties, is one containing about 3% to 9% chromium, 0.3%to 10% vanadium, 1% to 25% tungsten, up to 10% molybdenum, and thebalance essentially iron.

The foregoing ranges stated hereinabove may contain optionally up toabout 2% silicon, up to about 2% manganese, up to about 5% nickel, andup to about 15% cobalt.

One aspect of the invention resides in a powder metallurgy method ofproducing a wrought, high carbon tool steel alloy product characterizedby a substantially uniform composition throughout and a uniformdispersion of finely divided carbide substantially free from segregatesand/or aggregates. The method comprises providing a. batch of wrought,composite, mechanically alloyed, dense metal particles, substantiallyeach of said particles being comprised of a plurality of alloyableconstituents formulated to a desired high carbon tool steel compositionas set forth hereinbefore (note, for example, Tables I and ll), at leastone of the constituents being a compressible metal, such as iron. Thecomposite particles are characterized metallographically by an internalstructure comprising said constituents intimately united andinterdispersed. The batch of particles is then hot consolidated to awrought metal shape, whereby he wrought shape is characterizedsubstantially throughout by composition uniformity and a high degree ofdispersion uniformity of finely divided carbide in both the longitudinaland transverse directions.

One method of hot consolidating the batch of particles is to vacuum packcomposite particles in a mild steel can which is then welded shut,followed by hot extruding the canned powder at an elevated temperatureof at least about G-0 F., for example at 1900" F. to 2300 F.

In working with metals which melt above 1000 K., the heavy cold workimparted to the composite metal particle during its preparation isparticularly advantageous in the production of uniform compositions bysolid state diffusion at elevated temperatures. Observations on otheralloy systems have indicated that the heavy cold work increaseseffective diffusion coefficients in the product powder. This factor,along Iwith the intimate mixture in the product powder of metalfragments from the initial components to provide small interdilfusiondistances, promotes rapid homogenization and alloying of the productpowder upon heating to homogenizing temperatures. The

foregoing factors are of particular value in the production of complexhigh carbon high alloy tool steels in which diffusion tends to besluggish. Homogenization and/or annealing can be accomplished, forexample, during the heating of canned powders prior to extrusion.

One of the advantages of formulating compositions in n accordance withthe invention is that very little or no oxidation occurs during highenergy milling. However, unlike the kind of oxidation which occurs inconventional melting techniques, any extraneous oxides appear as nedispersoids and can be useful as dispersion strengtheners,

provided they are relatively chemically stable and temperatureresistant.

Thus, by producing composite metal powders in accordance with theforegoing, particles of substantially uniform composition are providedfrom which wrought metal products can be produced by hot consolidating abatch (e.g., a conned batch) of the particles to a desired shape, suchas by hot extrusion. Each particle is in effect a building blockexhibiting optimum metallographic uniformity, which uniformity iscarried forward into the final product unlike previous powdermetallurgical methods. In other words, in the case of thecarbide-forming constituents, these constituents are xed uniformly inposition in the particle so that upon interdiifusion at an elevatedtemperature, finely divided carbide particles are formed uniformlydispersed throughout the end product produced from the composite powder.

As illustrative of the use of the invention in producing high carbontool steel products, the following examples are given:

EXAMPLE I As indicated hereinbefore, one of the advantages of the methodprovided by the invention is that high carboncontaining alloys, such astool steels, may be produced whole avoiding the problem of formingcarbide dendrites or segregates which normally attend such alloys whenproduced by conventional melting and casting techniques. An example ofone composition is the rather complex high speed tool steel containingtungsten, 12% cobalt, 4% chromium, 2% vanadium, 0.8% carbon and thebalance essentially iron. In producing the composition, a mixture of theconstituents is placed in a high energy mill of the type illustrated inFIG. 2 containing a charge of approximately 1A inch hardened steel ballsat a ball-to-power volume ratio of about 20 to 1. The powder is drymilled at an impeller speed of 180l r.p.m. until composite metalparticles are formed cold worked to substantially saturated hardness andthe milling continued for a time suflicient to obtain an internalstructure within the particles in which the constituents are intimatelyunited and homogeneously interdispersed. The powder is thereafter vacuumsealed in a mild steel can and extruded at a temperature of about 2150F. at an extrusion ratio of 16 to l. The steel produced in this way,unlike to conventionally produced steel, will be free of carbidedendrites, segregates and/or aggregates.

An advantageous method in producing the foregoing composition is to makea blend of 28.6 grams of a 70% vanadium iron master alloy powder passing100 mesh, 57.2 grams of a chromium-30% iron master alloy powder passingmesh, 200 grams of tungsten powder of 10 micron Aaverage particle Size,120 grams of cobalt powder passing 325 mesh, 8 grams of graphite passing100 mesh and 586 grams of sponge iron powder of about 65 micron averagesize. This mixture is placed in the attritor mill and milled for about40 to 50 hours at about 180 r.p.m. using a charge of one-quarter inchhardened steel balls, the ball charge being sufficient to provide avolume ratio of balls to powder mixture of about 20 to 1. The 40 to 50hour milling is generally sufcient to produce composite particlescharacterized metallographically by a microstructure comprising asubstantially homogeneous interdispersion of all of the constituents.The resulting powder is hot extruded as described hereinabove.

The complex high speed steel of Example I is hardened by heating to atemperature of about 2350 F. for about 5 to l0 minutes followed by oilquenching to room temperatures, the cooled steel being thereaftersubjected to double tempering by a temperature of about 1050 F. forabout 2 hours.

EXAMPLE II In producing a wrought high carbon steel containing about0.85% carbon, about 0.2% chromium, about 0.7% manganese, about 0.3%silicon and the balance essentially iron, the following method isemployed:

A brittle high carbon master alloy is produced containing about 4.25%carbon, about 1% chromium, about 3.5% manganese, about 1.5% silicon andthe balance essentially iron. The master alloy is chill cast and thencrushed to pass 200 mesh. The crushed high carbon master alloy in anamount of 400 grams is mixed and uniformly blended with 1600 grams ofhigh purity sponge iron of about 65 microns in size. The mixture isplaced in the mill of the type described in Example I containing acharge of one-quarter inch diameter hardened steel balls at aball-to-powder ratio of about 18 to 1 by volume. The charge is drymilled yat an impeller speed of about r.p.m. until a highly cold workedcomposite metal powder is obtained after about 45 hours of millingcharacterized by a microstructure comprising a substantially homogeneousinterdispersion of all of the alloying constituents. The powder isthereafter annealed land employed in the hot extrusion of wrought toolsteel shapes. The composite powder is vacuum packed in a mild steel canwhich is welded shut. The can is heated to 2000 F. and then hot extrudedto a round rod at an extrusion ratio of about 16 to 1. The extrudedproduct is surface cleaned.

As stated hereinbefore, the extruded high carbon tool steel productsmade in accordance with the infvention are characterizedmetallographically by being free from carbide segregates and/oraggregates as well as exhibiting optimum response to heat treatment. Thesteel of Example II is hardened by oil quenching from an austenitizingtemperature of about 1450 F.. followed by tempering at a temperature ofabout 350 F.

EXAMPLE III A wrought semihigh speed steel composition can be producedin accordance with the invention having the following composition: about1.2% carbon, about 4% chromium. about 3% vanadium, about 4% molybdenum,about 0.3% manganese, about 0.3% silicon and the ball l. anceessentially iron. As in Example 1I, a brittle high carbon master alloyhaving the following composition is first produced: about 4.8% carbon,about 16% chromium, about 12% vanadium, about 12% molybdenum, about 1.2%manganese, about 1.2% silicon and the balance iron. The master alloy ischill cast and then crushed to pass 200 mesh. About 400 grams of thecrushed master alloy are mixed with 1200 grams of high purity spongeiron of about 65 microns in size. The mixture is placed in the mill ofthe type described in Example I containing a charge of one-quarter inchdiameter hardened steel balls at a ball-to-powder ratio of about 18 to 1by volume. The charge is dry milled at an impeller speed of about 175r.p.m. until a highly cold worked composite metal powder is obtainedafter about 48 hours of milling characterized by a microstructurecomprising a substantially homogeneous interdispersion of all of thealloying constituents. The powder is thereafter employed in the hotextrusion of wrought tool steel shapes. The composite powder is Vacuumpacked in a mild steel can which is welded EXAMPLE IV In producing awrought very high carbon, high speed steel containing about 2.5% carbon,about 4% chromium, about 7% vanadium, about 6% tungsten, about 2.5%molybdenum, about cobalt, and the balance essentially iron, thefollowing method was employed:

A powder blend consisting of 112.5 grams of graphite flakes passing 100mesh, 432 grams of a 70% vanadium- 30% iron master alloy powder passing100 mesh, 180 grams of chromium powder passing 100 mesh, 113 grams ofmolybdenum powder passing 325 mesh, 225 grams of cobalt powder passing325 mesh, 270 grams of tungsten powder of about 10 microns averageparticle size and 3191 grams of sponge iron powder of 65 microns size isplaced in a ball mill having a driven impeller and containing 200 poundsof 3A; inch diameter hardened steel balls and processed for hours at 246r.p.m. in a nitrogen atmosphere. At the end of this time a highly coldworked composite metal powder is obtained characterized by amicrostructure comprising a substantially homogeneous interdispersion ofall the alloying constituents. The composite powder is vacuum packed ina mild steel can which is welded shut. The can is heated to 2000 F. andthen hot extruded to a rod at an extrusion ratio of about 16 to 1. Thehardness of the extruded bar was found to be 62.5 Rc. The structure ofthe extruded material was very homogeneous with not more than 10% byvolume of the structure being segregated regions exceeding microns inminimum dimension. A tool or die made from the foregoing product ishardened by heating slowly to 1600 F., raising the temperature to 2200F., holding for 5 minutes and oil quenching. It is then double temperedfor 2 hours at 1000 F. followed by air cooling.

l2 The hardness of the material is found to be as high as 67 Rc.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and the appended claims.

I claim:

1. As a powder metallurgy article of manufacture, a wrought, high carbontool steel shape having a composition containing by weight about 0.7 to4% carbon, at least about 0.1% of at least one alloying element selectedfrom the group consisting of chromium, vanadium, tungsten andmolybdenum, and the balance essentially iron in an amount of at leastabout said wrought steel shape being characterized substantiallythroughout by composition uniformity, optimum heat treating response andby a high degree of dispersion uniformity of nely divided carbidessubstantially free of carbide segregates and aggregates such that lessthan 10 volume percent of segregated regions exceeding 25 microns inminimum dimension is present.

2. The article of manufacture of claim l, wherein the carbon contentranges from about 0.9 t0 3.5%.

3. The article of manufacture of claim 2, wherein said compositionincludes optionally up to about 2% silicon, up to about 2% manganese, upto about 5% nickel and up to about 15% cobalt.

4. The article of manufacture of claim 2, wherein the compositioncontains about 0.9 to 3.5% carbon, about 3 to 15% chromium, up to about20% vanadium, up to about 25% tungsten and up to about 12% molybdenum.

5. The article of manufacture of claim 4, wherein the compositioncontains about 0.9 to 3.5% carbon, about 3 to 9% chromium, about 0.3% to10% vanadium, about 1 to 25% tungsten, up to about 10% molybdenum, up toabout 2% silicon, up to about 5% nickel and up to about 15% cobalt.

6. The article of manufacture of claim 1 substantially free of carbidesegregates and aggregates such that less than 10 volume percent ofsegregated regions exceeding 10 microns in minimum dimension is present.

7. The article of manufacture of claim 2 containing at least 1% of atleast one alloying element selected from the group consisting ofchromium, Vvanadium, tungsten and molybdenum.

References Cited UNITED STATES PATENTS 2,853,767 9/1958 Burkhammer75-15X 3.245,763 4/1966 Fall 29-182.7 3,369,891 2/1968 Tarkan 29-182.8X3,369,892 2/1968 Ellis 29--182.7X 3,380,861 4/1968 Frehn.

CARL D. QUARFORTH, Primary Examiner B. H. HUNT, Assistant Examiner U.S.Cl. X.R.

P0-1050 UNITED STATES PATENT OFFICE W89) CERTIFICATE CE CORRECTIONPatent No. 3:591'349 Dated July 6, 1971 Inventor(s) JOHN STANWOODBENJAMIN It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as show-nbelow:

Col. l, line 57, for "insobluble" read insoluble.

Line 62, for "alloyng" read alloying.

Col. 2, line l0, for "partically" read practically.

Col. 8, Table II, for "Tungsten-cobalt types", last Same Table, for"Tungsten-molybdenum types" first number under "C" (first column) for"LOS-1.1

lo 0 1. l--o Signed and se aled thie 31st day of OctoberI 1972.

(SEAL) Attest:

HD1/JARD M.FLETGHER,JR. ROBERT GOTTSCHALK LAttesting Officer'Commissionerof Patents

